Not applicable.
Not applicable.
The present invention pertains to the removal of fluorine compounds from a multicomponent fluid and concentration of the removed fluorine compounds, and in particular to the concentration of fluorine compounds to be transferred to a separation process using at least one distillation column.
Fluorine compounds are widely used in the manufacture of semiconductors. These compounds can be costly and detrimental to the environment, if released.
In the manufacturing processes of some fluorine compounds, such as nitrogen trifluoride, nitrogen is produced as a reaction byproduct appearing as a diluent in the nitrogen trifluoride product stream. Dilution with inert gases, such as nitrogen or helium, is commonly performed for purging process equipment or to reduce reactivity of fluorine compounds during processing. In some cases, the dilution can be significant with a fluorine compounds content of 10% by volume, or less. The fluorine compounds may be recovered and purified by cryogenic distillation.
Diluted streams of fluorine compounds also are created in the process of manufacturing semiconductors. Because of the high costs of fluorine compounds and the detrimental effects of these compounds on the environment, it is desirable to recover and reuse these compounds. The fluorine compounds also may be purified by cryogenic distillation.
Cryogenic distillation equipment used to recover and purify fluorine compounds from a diluent gas could be significantly reduced in size if the fluorine compounds were not so diluted. Such a reduction in size would reduce the capital and operating costs of the distillation equipment.
Also, the flow rates and concentrations of streams of diluted fluorine compounds may vary. For improved operability, it is desirable to stabilize the feed to a continuous distillation system. For example, this may be done by creating a storage capacity for the feed. This storage capacity should be cost effective, since the volume of the concentrated feed is reduced in comparison with the volume of the original, diluted feed.
If a batch distillation is used, it is necessary to store the feed mixture for the next available batch. The size reduction of the feed storage could be achieved by initial concentration of the fluorine compounds in the feed.
Initially, since the flow rates of fluorine compounds from reactors and of fluorine compounds recovered from semiconductor fabrication plants usually were relatively low, the issue of pre-concentrating diluted streams was not significant. With the development of the electronic industry, the demand for fluorine compounds has increased significantly, causing increases in production rates and use. Now, the issue of pre-concentrating fluorine compounds from dilute streams is economically and environmentally attractive.
U.S. Pat. No. 5,832,746 (Nagamura) discloses a cryogenic distillation process for the purification of nitrogen trifluoride without any initial pre-concentration step. In U.S. Pat. Nos. 5,502,969 (Jin, et al.) and 5,771,713 (Fisher) an absorption column is used prior to distillation to absorb fluorine compounds in a xe2x80x9cwash liquidxe2x80x9d, for example perfluoropropane (C3F8), propane, ethane or a mixture thereof. The disadvantage of adding a new component (i.e., a wash liquid) is that this component eventually must be separated from the fluorine component, for example by cryogenic distillation. Also, a wash liquid containing fluorine compounds may absorb some diluent gas and other impurities. However, the wash liquid keeps certain fluorine components from solidifying and reduces the vapor pressure of the fluorine compounds, as described in U.S. Pat. No. 5,626,023 (Fisher, et al.).
U.S. Pat. No. 5,150,577 (Mitchell, et al.) discloses a system and method for recovering and purifying halocarbon compounds, particularly halon. Halocarbon compounds diluted in nitrogen are partially liquefied by indirect heat exchange with a cooling liquid, for example liquid nitrogen. A liquefied fraction is collected and the vapor fraction (comprising nitrogen and a small amount of halocarbons) is vented through a carbon adsorber, where the organic halocarbon vapor is adsorbed and effectively removed from the nitrogen gas. The adsorbed halocarbons are periodically recovered from the adsorber by vacuum desorption and recycled back to the feed gas.
Other patents related to fluorine compounds deal with purification of fluorine compounds from certain components by adsorption (U.S. Pat. No. 5,069,887xe2x80x94removal of CF4) (Suenago, et al.) or by chemical decomposition (U. S. Pat. No. 5,183,647xe2x80x94removal of N2F2) (Harada, et al.).
U.S. Pat. No. 5,779,863 (Ha, et al.) discloses a method for separating and purifying perfluorocompounds (PFC""s) using a cryogenic distillation system.
It is desired to reduce the capital and operating costs of cryogenic distillation systems for recovering and purifying fluorine compounds by pre-concentrating the compounds in a diluent gas prior to distillation.
It is further desired to improve the operability of continuous cryogenic distillation systems by introducing, in a cost-effective way, additional storage capacity for a concentrated feed stream, which stabilizes the flow rate and composition of the fluorine compounds entering such a distillation system.
It is still further desired to have a process which provides a cost effective (reduced in size) feed storage for batch distillation.
It is still further desired to have a process to pre-concentrate fluorine compounds in a safe and effective manner, without any releases to the environment.
It also is desired to have a process for removing from a multicomponent fluid one or more fluorine compounds present at a first concentration in said multicomponent fluid and for concentrating said fluorine compounds which overcomes the difficulties and disadvantages of the prior art to provide better and more advantageous results.
The present invention is a process for removing from a diluted gas one or more fluorine compounds present in a first concentration in the diluted gas and for concentrating the fluorine compounds. This is done by directly contacting the diluted gas with a cryogenic liquid having a temperature less than or equal to aboutxe2x88x92150xc2x0 F., thereby producing a vapor lean in the fluorine compounds and a liquid enriched with the fluorine compounds at a second concentration. The second concentration is greater than the first concentration.
There are a variety of variations of the invention. For example, the diluted gas contains at least one diluent compound selected from the group consisting of nitrogen, oxygen, argon and helium. Each of the fluorine compounds contains at least one fluorine atom. Also, the diluted gas may contain at least one fluorine compound from the group consisting of nitrogen trifluoride (NF3), freonxe2x88x9214 or tetrafluoromethane (CF4), freon xe2x88x9223 or trifluoromethane (CHF3) and hexafluoroethane (C2F6).
In another variation, the first concentration is preferably less than or equal to about 50 mole % and more preferably less than or equal to about 20 mole %.
In another variation, the cryogenic liquid is selected from the group consisting of liquid nitrogen, liquid oxygen, liquid argon, liquid helium and mixtures thereof.
In another variation, the diluted gas is directly contacted with the cryogenic liquid in a heat and mass transfer device having a top and a bottom. In a variant of this variation, the cryogenic liquid is introduced at the top of the heat and mass transfer device and the diluted gas is introduced at the bottom of the heat and mass transfer device.
In yet another variation, the diluted gas is indirectly heat exchanged with the vapor lean in the fluorine compounds.
In another aspect of the invention, the liquid enriched with the fluorine compounds at the second concentration is transferred to a separation unit comprising at least one distillation column.
Another embodiment of the invention is a process for removing from a multicomponent fluid one or more fluorine compounds present at a first concentration in the multicomponent fluid and for concentrating the fluorine compounds. The process uses a vessel having a top, a bottom, and a heat and mass transfer zone between the top and the bottom. The process comprises multiple steps. The first step is to feed the multicomponent fluid to the vessel at a first feed location below the heat and mass transfer zone. The second step is to feed a cryogenic liquid to the vessel at a second feed location above the heat and mass transfer zone, the cryogenic liquid having a temperature less than or equal to aboutxe2x88x92150xc2x0 F. The third step is to remove a vapor lean in fluorine compounds from the top of the vessel. The fourth step is to remove a liquid enriched with fluorine compounds and a second concentration from the vessel at a location below the first feed location, the second concentration being greater than the first concentration.
This embodiment also has a number of variations. For example, the multicomponent fluid contains at least one diluent component selected from the group consisting of nitrogen, oxygen, argon, and helium. Each of the fluorine compounds contains at least one fluorine atom. Also, the multicomponent fluid may contain at least one fluorine compound from the group consisting of nitrogen trifluoride (NF3), freonxe2x88x9214 or tetrafluoromethane (CF4), freonxe2x88x9223 or trifluoromethane (CHF3) and hexafluoroethane (C2F6).
In another variation, the first concentration is preferably less than or equal to about 50 mole % and more preferably less than or equal to about 20 mole %.
In addition, there are other variations. For example, the cryogenic liquid is selected from the group consisting of liquid nitrogen, liquid oxygen, liquid argon, liquid helium, and mixtures thereof. In one variation, the multicomponent fluid is directly contacted with the cryogenic liquid in the heat and mass transfer zone. In another variation, the multicomponent fluid is indirectly heat exchanged with the vapor lean in the fluorine compounds.
In another aspect of the invention, the liquid enriched with the fluorine compounds in a second concentration is transferred to a separation unit comprising at least one distillation column.